Centrifugal grinding mills have been widely applied for the comminution of solid particles, in particular for the size reduction of mineral ores. A feature of centrifugal grinding mills is that the centrifugal acceleration imparted to the mill contents as a consequence of their gyrating motion has the effect of greatly enhancing the rate of comminution compared with conventional tumbling mills, which are limited by gravitational acceleration.
A particular type of centrifugal grinding mill to which this invention relates is that in which the grinding chamber axis is inclined to and intersects the axis of rotation of the chamber, which is characteristic of nutating motion. For the purpose of this specification nutating motion of a machine element relative to a fixed frame is defined as the motion of the element, an axis of which intersects with and traces out a conical surface about a stationary axis of the fixed frame. In the general case, the nutating element has a net rotational motion about its axis, relative to the fixed frame. A special case of nutating motion is one in which the nutating element has a net irrotational motion.
FIGS. 1 and 2 show cross sections through known centrifugal grinding mills of this type, such as that described in the specifications accompanying International Patent Application No. PCT/AU99/00695, and Australian Patent No. 568949. The figures shows a grinding chamber 104, having an axis of symmetry 102, which rotates about a fixed axis 101, and intersects therewith at a point of nutation symmetry 103. The chamber is constrained to perform nutating motion by the engagement of complementary annular bearing surface pairs 109 and 111, and 108 and 110, which together form a spherical bearing symmetrical about the point of nutation symmetry 103, and which limit the amplitude of nutating motion. Feed material, in the form of coarse granular material 131, as a suspension of coarse particles in a fluid, or as a combination of both of these alternatives, enters the grinding chamber from the feed passage 105, and discharges from the opposite extremity of the grinding chamber via openings 106, as a fine granular product. In the embodiment shown in FIG. 1, the grinding chamber 104 is driven with a nutating motion about stationary axis 101 by multiple pistons 159 which are driven in phased sequence and bear against a flanged extension of the grinding chamber located about the point of nutation symmetry 103. In the embodiment shown in FIG. 2, the grinding chamber is driven with a nutating motion by shaft 114, which connects with electric motor 115 at one end, and engages with the end of chamber 104 through an eccentric stub shaft 119, attached to the other end.
It is a characteristic of these types of grinding machines that production of coarse and intermediate sizes product material requires the use of grinding media 132 having a size of typically 5 to 20 mms spherical diameter, which is larger than the size of discharge openings 106 in the wall of the chamber 104. Particles of grinding media are thus contained within the chamber, and only particles of feed material, or worn grinding media, having a size smaller than the openings 106, can discharge from the chamber. Production of very fine product material, typically having 80 percent finer than 40 microns, requires the use of correspondingly fine grinding media, typically 1 to 5 mm spherical diameter, for minimum energy consumption. The use of correspondingly small discharge openings 106 in the periphery of the grinding chamber 104 is not feasible. It has been found from experience that centrifugal grinding machines having discharge openings of the type shown in FIGS. 1 and 2 are unsuited to production of very fine product. As the size of discharge opening is reduced, the likelihood of blockage by oversize particles increases inversely with the size of discharge opening. Furthermore, the very high surface pressures at the internal surface of the grinding chamber is not compatible with the structural or wear requirements of very fine discharge apertures.